The present invention relates to a new hydrocarbon conversion process wherein a hydrocarbon feed is contacted with a highly selective dehydrocyclization catalyst in a reaction vessel to produce a product stream; then the paraffins in the product stream are extracted and recycled to the reaction vessel.
Catalytic reforming is well known in the petroleum industry. It involves treating naphtha fractions to improve the octane rating by producing aromatics. The hydrocarbon reactions occurring during reforming operation include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of acyclic hydrocarbons to aromatics dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. Hydrocracking reactions should be particularly minimized during reforming as they decrease both the yield of gasoline boiling products and the yield of hydrogen.
Because of the demand for high octane gasoline for use in motor fuels, etc., extensive research is being devoted to developing improved reforming catalysts and catalytic reforming processes. Catalysts for reforming processes must be able to produce high yields of liquid products in the gasoline boiling range containing large concentrations of high octane number aromatic hydrocarbons and low yields of light gaseous hydrocarbons. The catalysts should possess good activity in order that low temperatures can be used to produce a quality product. The catalysts should also either possess good stability, in order that the activity and selectivity characteristics oan be retained during prolonged periods of operation, or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalysts comprising platinum, for example, platinum supported on alumina, are widely used for reforming of naphthas.
Some have proposed the use of certain molecular sieves such as X and Y zeolites, which have pores large enough for hydrocarbons in the gasoline boiling range to pass through. However, catalysts based upon these molecular sieves have not been commercially successful.
In conventional reforming, the hydrocarbons to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of about 450.degree. C. to 550.degree. C. and pressures of from 50 to 500 psig. Part of the hydrocarbons are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The catalysts hitherto used have given fairly satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a type L zeolite are more selective with regard to the dehydrocyclization reaction and produce excellent results with C.sub.6 -C.sub.8 paraffins.
A major recent development was a new dehydrocyclization catalyst which comprises a large-pore zeolite, a Group VIII metal, and an alkaline earth metal. This catalyst has a superior selectivity for dehydrocyclization. This selectivity is so high that most of the paraffins that are not dehydrocyclized remain as paraffins in the product stream, and reduce the octane rating of the resulting product.